1. Field of the Invention
The present invention pertains to the manufacture of solid articles from particulate materials such as powdered tetrafluoroethylenes and similar thermosetting plastics, as well as ceramics, metals and other materials. The individual particles of such powdered plastics are characterized by numerous surface irregularities. Thus, when a compressive force is exerted on a quantity of such a powder, these irregularities are caused to interlock with one another to form a solid article. When the compressive force is applied by fluid pressure via a diaphragm, such a process is known as "isostatic molding." The article formed by such a pressing process, while solid and integral, is porous and/or brittle. However, if this preliminary article, known as a "preform," is heated to a suitable temperature, the plastic sets, resulting in a finished article which is neither porous nor brittle but rather extremely strong and substantially homogenous in texture. The same basic techniques may also be employed in the formation of articles from particulate metals, ceramics, and other materials.
2. Description of the Prior Art
The basic pressing and sintering technique described above is acceptable for the manufacture of many relatively simple types of plastic articles. However, problems are encountered where the finished article is complicated by a convoluted or otherwise irregular configuration, interconnection between the plastic portion of the article and another portion of metal or some other substance, and/or an intended use in an environment in which extremely good physical properties are needed.
For example, with certain types of articles having complex convoluted or otherwise irregular configurations, it has heretofore been considered impossible or at least economically unfeasible to form the article into such convoluted configuration from the original powdered plastic. The problems of providing properly shaped removable mold pieces is particularly acute where, for example, the article is a sleeve or other hollow body which is further internally convoluted. Accordingly, it has been customary to first form a relatively simple body, such as a cylindrical sleeve, by the pressing and sintering methods, and then physically deform the finished cylindrical sleeve into the desired convoluted configuration. In using such a technique, it is necessary to stretch the finished plastic, and the resulting convoluted article thus has a thinner cross-section through the areas which have been so stretched. While these thin-sectioned areas are therefore the weakest points of the finished article, they may be located at precisely the points at which the greatest strength is needed, an obvious disadvantage.
In connection with articles comprised of two interconnected bodies, e.g. one of a thermosetting plastic and one of metal, having incompatible coefficients of thermal expansion, problems may occur in the sintering stage of the manufacturing process. For example, the plastic may tend to expand more than the metal. Then, if the metal and plastic are interengaged in such a way that the metal inhibits expansion of the plastic at the interface of the two, the plastic may buckle along an exposed or free surface distal the metal portion of the article.
Still another problem is that of producing articles which require superior physical properties. In the past, this has been done by actively exerting pressure on the preform as it is being sintered. However, the use of this technique, commonly known as "pressure cooking," is relatively expensive and increases the cost of the finished article.
The same or similar problems may be encountered in the manufacture of articles from particulate materials other than plastics.